Gas metal arc welding (GMAW), sometimes referred to by its subtypes metal inert gas (MIG) welding or metal active gas (MAG) welding, is a semi-automatic or automatic arc welding process in which a continuous and consumable wire electrode (“welding wire”) and a shielding gas are fed through a welding gun.
FIGS. 1 and 2 illustrate the basic design of a conventional industrial GMAW system. As shown in these figures, GMAW system 10 includes electrical power source 12, wire drive assembly 14, shielding gas supply system 16, and cable assembly 18 for carrying electrical power, welding wire and shielding gas to a workpiece 20 to be welded. Wire drive assembly 14 typically includes reel stand 22 for carrying a spool 24 of a continuous, consumable wire electrode as well as drive mechanism 26 including one or more drive wheels (not shown) for driving welding wire from spool 24 through cable assembly 18 to workpiece 20. Meanwhile, shielding gas supply system 16 normally includes shielding gas source 28 and gas supply conduit 30 in fluid communication with cable assembly 18.
As illustrated especially in FIG. 2, cable assembly 18 typically takes the form of an elongated flexible cable 32 attached on one end to power source 12, wire drive assembly 14 and gas supply system 16 and on its other end to weld gun 34. As illustrated in FIG. 3, which is a radial cross-section of flexible cable 32, this flexible cable normally includes an electrical cable 34 for providing welding electrical power to the contact tip of weld gun 40, gas conduit 36 for transporting shielding gas, and flexible sheath 48 for housing the welding wire.
In practice, flexible cable 32 is normally at least 10 feet (˜3 m) long, more typically at least 15 feet (˜4.6 m), at least 20 feet (˜6.1 m), at least 25 feet (˜7.6 m), or even at least 30 feet (˜9.1 m) long, so that electrical power source 12, wire drive assembly 14 and shielding gas supply system 16 can remain essentially stationary while weld gun 34 is moved by hand to various different locations. In addition, flexible cable 32 is normally made as flexible as possible, since this provides the greatest degree of flexibility in terms of moving and positioning weld gun 34 in any desired location. So, for example, flexible cable 32 is normally made flexible enough so that it can make relatively tight bends, such as being coiled into multiple revolutions, as illustrated in FIG. 2.
In order to prevent welding wire from snagging inside flexible cable 32, the welding wire is threaded through the interior of a flexible sheath 48. Normally, this flexible sheath is made from a metal wire tightly wound in a spiral whose inside diameter is only slightly larger than the outside diameter of the welding wire, since this structure provides a high degree of flexibility in flexible cable 32 while simultaneously preventing contact between the welding wire and other components inside the flexible cable.
Because of the length and flexibility of elongated flexible cable 32, it often takes a comparatively great amount of force to drive welding wire from spool 24 through cable assembly 18 onto workpiece 20. Therefore, it is common practice in industry to coat the welding wire with a wire feeding lubricant for reducing the coefficient of friction between its external surfaces and the internal surfaces the flexible sheath through which it passes. Sodium and calcium based soaps, e.g., sodium stearate and calcium stearate, are most commonly used for this purpose. Alternatively, certain solid particulate materials, such as MoS2, WS2, ZnO (normally together with WS2), graphite and/or PTFE (Teflon), have also been used for this purpose.
A real advantage of soap based feeding lubricants is lubricity or feedability, i.e., the ability of the lubricant to enable transport of the welding wire from its supply spool to the weld gun assembly with minimal force and as smoothly as possible, especially when feeding is interrupted with numerous starts and stops. This is because soap-based lubricants are generally soft and pliable in the sense that, under the temperatures, shear stresses and other localized conditions encountered during feeding, soap-based lubricant tend to soften or plasticize into slippery, pliable, semi-solid materials. Generally speaking, solid particulate feeding lubricants do not offer the same superior level of feedability, because they remain in hard, solid particulate form during the feeding operation.
Another advantage of soap based feeding lubricants is arc stability, i.e., the ability of the lubricant to promote a uniform, uninterrupted arc between the electrode tip and the workpiece being welded. A stable arc promotes formation of a uniform weld bead, because it is the arc that melts the electrode. Generally speaking, particulate feeding lubricants do not offer the same superior arc stability as soap based lubricants, again, because they remain in hard, solid particulate form during the feeding operation.
Although soap-based feeding lubricants are superior to solid particulate feeding lubricants in terms of feedability and arc stability, they are inferior in terms of hydrogen contamination of the weld metal that is formed, especially when cored welding electrodes are used. Excess hydrogen contamination can lead to increased cracking, and so using cored welding electrodes is often avoided in military and other applications where high strength weld joints are needed. See, for example, D. D. Harwig et al., Effects of Welding Parameters and Electrode Atmospheric Exposure on the Diffusible Hydrogen Content of Gas Shielded Flux Cored Arc Welds, Welding Research Supplement, September 1999, p 314-321.
Thus, it will be appreciated that there is an inherent problem with conventional welding wire feeding lubricants—those which provide the desired degree of feedability (lubricity) for use in GMAW equipment generate unacceptable amounts of hydrogen contamination while those which produce acceptably low levels of hydrogen contamination provide insufficient feedability (lubricity).